The present invention relates to a method and apparatus for cooling an optical fiber during the drawing process of said fiber.
In the production of optical fibers, a glass preform is first prepared according to known technologies, comprising VAD (vapor axial deposition), OVD (outside vapor deposition) and MCVD (modified chemical vapor deposition), by depositing a soot of glass particles; said soot-glass preform is then consolidated before drawing the fiber.
The optical fiber is obtained from the consolidated preform by heating the bottom end of the preform at its softening temperature into a so-called xe2x80x9cdrawing furnacexe2x80x9d and drawing the fiber from said softened preform under controlled conditions, according to known procedures. Upon cooling, the glass solidifies into the optical fiber, which is very fragile. Thus, during the drawing phase, before collecting it, the fiber is normally coated with one or more layers of synthetic coating materialxe2x80x94preferably two layersxe2x80x94for instance urethane-acrylate resins) for protecting it.
In general the coating of the fiber is performed by passing the fiber through a xe2x80x9ccoating-diexe2x80x9d, which contains a liquid resin. The fiber, which in general has a temperature of about 2000xc2x0 C. at its outlet from the drawing furnace, has to be cooled before entering into the coating-die at a temperature compatible with the one of the coating application technique, (in general below 100xc2x0 C., preferably at about 25xc2x0 C.-60xc2x0 C.) for avoiding possible inconveniences during the coating phase due to the high temperature of the fiber, resulting in irregular deposition of the coating layer. By increasing the draw speed, the fiber requires longer distances for cooling down to a temperature suitable for the coating application. For instance, as reported in U.S. Pat. No. 4,437,870, a distance of 120 cm is required for natural cooling a fiber with a 125 xcexcm diameter from 1780xc2x0 C. down to 50xc2x0 C., with a draw speed of 0.75 m/sec; when the draw speed is increased up to 5 m/sec, a 800 cm cooling distance is required. With the increasing rate of the drawing speed the distance between the drawing furnace and the coating die would increase too much when only a natural cooling is applied; it has thus been suggested to employ cooling means for force-cooling the fiber at a suitable temperature for the coating application, allowing to employ shorter cooling distances.
U.S. Pat. No. 4,437,870 discloses an apparatus for cooling the fiber which consists of a vertical tube through which the fiber is drawn, said tube being provided at its bottom end with a cylindrically-shaped porous member. Cooling gas is supplied into a chamber surrounding the porous member, and then, through said porous member, it flows upwardly along the fiber to the top of the cooling tube. A chamber containing liquefied gas (nitrogen) surrounds the cooling tube. According to an alternative embodiment the fiber is drawn through a vertical tube, which may be surrounded by a layer of insulating material, having an annular opening at its bottom through which cooling gas emanates into the tube, flowing upwardly to the top of the cooling tube.
U.S. Pat. No. 4,514,205 discloses an apparatus for cooling the fiber consisting of a cooling tube surrounding the fiber, which tube is centrally disposed in a reservoir containing liquefied gas. The cooling gas flows first through a coil disposed into the reservoir, thus being cooled by the liquefied gas contained in said reservoir, and then into the cooling tube axially along the fiber.
U.S. Pat. No. 4,913,715 discloses a cooling apparatus wherein the fiber is drawn through a forced-cooled double-walled tube. The inner space of the tube, through which the fiber passes, contains a gas with good heat-transporting properties, having a flow which is reduced but sufficient to prevent the penetration of the surrounding atmosphere into the tube and to compensate loss of gas. According to the method disclosed in this patent, the fiber is thus cooled substantially by heat transfer to the cooled wall by means of the heat-transporting gas surrounding the fiber.
U.S. Pat. No. 4,966,615 discloses a cooling tube surrounded by a cooling jacket. A number of ring-shaped partition plates, spaced from each other, are mounted within the tube. The partition plates allows breaking the laminar flow of the gas through the tube, in order to increase the heat transfer between gas and fiber.
U.S. Pat. No. 4,838,918 discloses a method for cooling an optical fiber wherein said fiber is passed between two parallel plates cooled with nitrogen, a laminar flow of inert gas being directed onto said fiber along a plane centrally located between said plates, said laminar flow being generated by a xc2xd inch tube provided on its surface with a number of holes of {fraction (1/16)} inches diameter, spaced one inch apart from each other.
EP 319,374 discloses a cooling device comprising a pair of parallel plates between which the fiber is passed, said plates being optionally cooled for absorbing the heat radiating from the fiber, and a pair of vertically oriented copper tubes delivering a laminar flow of room temperature nitrogen gas between the parallel plates to surround the downwardly moving fiber.
GB 2,287,244 discloses the cooling device comprising an elongated water-cooled body member provided with a through hole opening out abruptly into a succession of spherical chambers in which a cyclonic flow of gas is induced by the tangential injection of helium, preferably with opposite cyclonic rotation in successive chambers.
DE 4,412,563 discloses a cooling device having a plurality of gas-flow openings in a structure surrounding longitudinal axis of the fiber, said plurality of gas-flow being positioned at respective different heights along said structure.
U.S. Pat. No. 4,664,689 relates to a method and apparatus for rapidly cooling an optical fiber comprising passing the optical fiber through an enclosure having a flat back internal surface, the walls of said enclosure having symmetrically oriented perforations or other symmetrically oriented means of passing cryogenic gas through the walls to contact the optical fiber within the enclosures.
The applicant has noticed that the above cooling devices and methods have some drawbacks in their use, in particular as they can not be easily adapted to the variations of the drawing conditions.
Furthermore, the applicant has also observed that with conventional cooling methods employing an axial flow of cooling gas, the possibility to increase the drawing speed is also limited from a critical value of the flow rate of the cooling gas (depending on its initial temperature and on the length of the tube), above which there is a saturation of the cooling efficiency of the gas, with no substantial increase in the cooling capacity of the gas.
In addition, in the method and apparatus disclosed in U.S. Pat. No. 4,838,918, the applicant has observed that the efficiency of the cooling of the fiber can be reduced by the fact that the cooling gas is provided by means of small holes spaced from each other and that there are no means for effectively removing the inert gas from the apparatus.
It has now been found that according to the present invention, the efficiency of the cooling of the optical fiber can be improved by passing said fiber through a hollow elongated body, said body being provided with at least a first longitudinal opening and at least a second longitudinal opening, both said openings being provided substantially on the whole length of said elongated body and said second opening being positioned at substantially the opposite side with respect to said first opening, wherein the cooling gas is passed through said first opening, directed onto the fiber and removed from the opposite second opening. Accordingly, the cooling method of the invention provides a flow of cooling gas which is substantially transversal with respect the longitudinal axis of the drawn fiber, for the whole path of the cooling gas inside the elongated hollow body. In the present description the wording xe2x80x9cflow direction substantially transversal with respect to the longitudinal axis of the fiberxe2x80x9d is intended to encompass any condition in which the cooling gas flows transversally to the longitudinal axis of the fiber from one side to the other of the elongated hollow body through which the fiber passes. Preferably, the direction of the transversal flow of the cooling gas is substantially perpendicular with respect to the longitudinal axis of the fiber.
One aspect of the present invention thus relates to a method for cooling an optical fiber which comprises:
passing said fiber through a hollow elongated body, said elongated body being provided with at least a first longitudinal opening and at least a second longitudinal opening;
flowing a cooling gas through said first opening, wherein the flow direction of the cooling gas is substantially transversal with respect the longitudinal axis of the fiber;
removing said cooling gas from the hollow elongated body through said second opening.
According to a preferred embodiment, said openings are provided on at least half of the total length of said elongated body. Preferably, the length of said openings corresponds to at least 75% of the total length of said elongated hollow body; in particular, the length of said openings ranges from about 80% to about 95% of the total length of said elongated hollow body. According to a preferred embodiment, said second opening is positioned substantially on the opposite side of said elongated body with respect to said first opening. Furthermore, the cooling gas is preferably forcibly removed from said hollow elongated body through said second longitudinal opening.
According to a preferred embodiment, the method of the present invention comprises:
introducing a cooling gas at a predetermined temperature into a first hollow space defined by an inner and an outer wall of a first double-walled half-tube;
flowing said cooling gas from said first hollow space through at least one longitudinal opening, provided on the inner wall of said first double-walled half-tube, into a central chamber defined by the inner wall of said first double-walled half-tube and by the inner wall of a second double-walled half-tube, in order to cool an optical fiber being passed through said central chamber;
flowing the cooling gas from said central chamber through at least a second longitudinal opening, provided on the inner wall of said second double-walled half-tube, into a second hollow space defined by the inner and the outer wall of said second double-walled half-tube;
removing the cooling gas from said second hollow space.
According to a preferred embodiment the cooling method of the present invention comprises:
subjecting at least a first portion of the fiber to a substantially transversal flow of cooling gas in a first direction; and
subjecting at least a second portion of said fiber to a second substantially transversal flow of cooling gas in a second direction, said second direction preferably being substantially opposite with respect to the first one.
A further aspect of the present invention relates to a cooling device for cooling an optical fiber drawn from a softened preform, said device comprising a hollow elongated body, said hollow elongated body having at least one wall defining an internal elongated space through which the drawn fiber passes wherein the at least one wall of said hollow elongated body is provided with at least one longitudinal opening through which a cooling gas is introduced into the hollow body and at least one longitudinal opening through which said cooling gas is removed from said hollow body, said openings having respective orientations with respect to the path of the optical fiber through the hollow body, such that the flow direction of the cooling gas results substantially transversal with respect the longitudinal axis of the fiber passing through said hollow elongated body.
Typically, said hollow elongated body is a tube.
According to a preferred embodiment, the above mentioned hollow elongated body is a double-walled tube comprising an inner and an outer wall defining a first hollow space, wherein:
the inner wall of the tube defines a second hollow space corresponding to the central part of the tube through which the fiber passes;
said inner wall is provided with the at least one longitudinal opening through which the cooling gas from the first hollow space is introduced into the central part of the tube and at least one longitudinal opening through which said cooling gas is removed from the central part into the first hollow, space.
According to a preferred embodiment, said double-walled tube comprises two separate halves joined together to form the tube, the inner walls of the two halves delimiting a central part of the cooling tube through which the fiber passes, each half having the inner wall provided with at least one longitudinal opening. The cooling gas flows from an inlet into the hollow space defined by the inner and outer wall of the first half and then, through the at least one slot on the inner wall of said first half, into the central part of the cooling tube onto the fiber; then, the cooling gas flows from the central part of the cooling tube through the at least one slot on the inner wall of second half into the hollow space defined by the two walls of the second half and is removed through an outlet connected to said hollow space of the second half.
A further aspect of the present invention relates to a cooling system for cooling an optical fiber which comprises:
a cooling apparatus in which a cooling gas is flown onto the optical fiber with a substantially transversal flow direction with respect the longitudinal axis of the fiber; and
a regeneration unit, connected to said cooling apparatus.
Preferably, said regeneration unit comprises at least a purification or a refrigeration device for purifying and/or refrigerating the cooling gas.